Electrical antifuses and fuses are used in the semiconductor industry to store non-erasable information. Once programmed, the programmed state of a fuse or an antifuse does not revert to the original state on its own; that is, the programmed state of the fuse or the antifuse is not reversible. For this reason, electrical fuses and antifuses are called One-Time-Programmable (OTP) memory elements. Thus, fuses and antifuses are conducive to the manufacture of a programmable read only memory (PROM). Programming or lack of programming constitutes one bit of stored information in a fuse or an antifuse. The difference between a fuse and an antifuse is the way the resistance of the memory element is changed during the programming process. A semiconductor fuse has a low initial resistance state that may be changed to a higher resistance state through programming, i.e., through electrical bias conditions applied to the fuse. In contrast, a semiconductor antifuse has a high initial resistance state that may be changed to a low resistance state through programming.
Various methods of implementing an antifuse in a semiconductor structure have been known in the prior art. In general, an antifuse includes one insulating layer sandwiched between two electrically conducting plates. In some cases, the insulating layer is a dielectric layer such as silicon dioxide or silicon nitride. In some other cases, the insulating layer comprises a stack of multiple layers including at least one silicon nitride layer and at least one silicon dioxide layer such as an oxide/nitride/oxide (ONO) stack. In a typical antifuse, the three components of the antifuse, i.e., the first electrically conducting plate, the insulating layer, and the second conducting plate, are built in a vertical stack. By supplying a large voltage difference across the two electrically conducting plates, a dielectric breakdown is induced and a current path between the two electrically conducting plates is formed, whereby the high resistance state of the antifuse changes to a low resistance state. Various materials may be used for each of the two electrically conducting plates. Improvements upon the basic structure are also known in the prior art. In one example, U.S. Pat. No. 6,853,049 utilizes a silicide for one electrically conducting plate and polysilicon for the other electrically conducting plate. In another example, U.S. Pat. No. 6,750,530 provides a mechanism for lowering the antifuse programming voltage by providing a resistive heating element adjacent to, but not in contact with the antifuse.
Incorporation of electrical antifuses or fuses into a semiconductor chip requires an external sensing circuitry located outside an array of OTP memory elements for detecting the status of each of the OTP memory elements, i.e., electrical antifuses or electrical fuses. Typically, such circuitry needs to supply a high enough voltage to differentiate the voltage output from each OTP memory element in an array. However, such a circuit design requires that the supplied voltage does not droop substantially across the array as well as requiring that the sense output voltage from each of the OTP memory elements does not droop significantly beyond the noise margin of the sense circuitry. In addition, typical external sensing circuitry requires transistors of substantial size to deliver a large current through the wiring of the array and to insure sufficient signal development during sensing operations.
Incorporating a sensing mechanism into the antifuse structure can reduce the current and noise margin requirements for the external sensing circuitry. However, adding sensing transistors to each antifuse element typically increases the size of the OTP memory element significantly as exemplified in U.S. Pat. No. 6,927,997 to Lee et al, wherein three transistors are required to constitute one antifuse memory element.
Therefore, an antifuse structure with a compact layout area and a built-in sensing mechanism is desired.
Furthermore, an antifuse structure with a wide built-in sense margin within such a built-in sensing mechanism is also desired.
Also, a general challenge in the incorporation of electrical antifuses into semiconductor chips is a relatively high voltage required to program the dielectric material within an electrical antifuse. This is especially important since semiconductor chips provide only a relatively low supply voltage in many applications such as communication and mobile computing. An electrical antifuse with a low programming voltage is therefore desired.